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CIHM/ICMH 
Microfiche 


CIHM/ICMH 
Collection  de 
microfiches. 


Canadian  Institute  for  Historical  Microreproductions  Institut  Canadian  de  microraproductions  historiques 

1980 


Technical  Notes  /  Notes  techniques 


The  Institute  has  attempted  to  obtain  the  best 
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ddfauts  susceptibles  de  nuire  d  la  quality  de  la 
reproduction  sont  notds  ci-dessous. 


D 
D 
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Coloured  maps/ 

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Pages  discoloured,  stained  or  foxed/ 
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Reliure  serri  (peut  causer  de  I'ombre  ou 
de  la  distortion  le  long  de  la  marge 
intdrieure) 


n 


n 


Coloured  pages/ 
Pages  de  couleur 


Coloured  plates/ 
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Show  through/ 
Transparence 


Pages  damaged/ 
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Additional  comments/ 
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n 

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Seule  Edition  disponible 


Bound  with  other  material/ 
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Cover  title  missing/ 

Le  titre  de  couverture  manque 


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D 
D 
D 


Pagination  incorrect/ 
Erreurs  de  pagination 


Pages  missing/ 
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Maps  missing/ 

Des  cartes  gdographiques  manquent 


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The  images  appearing  here  are  the  best  quality 
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plus  grand  soin,  compte  tenu  de  ia  condition  et 
de  la  nettet6  de  I'exemplaire  film6,  et  en 
conformity  avec  les  conditions  du  contrat  de 
filmage. 


The  last  recorded  frame  on  each  microfiche  shall 
contain  the  symbol  ^^>  (meaning  CONTINUED"), 
or  the  symbol  V  (meaning  "END"),  whichever 
applies. 


Un  des  symboles  suivants  apparaTtra  sur  la  der- 
niAre  image  de  cheque  microfiche,  selon  le  cas: 
le  symbols  — ►  signifie  "A  SUIVRE".  le  symbols 
V  signifie  "FIN". 


The  original  copy  was  borrowed  from,  and 
filmed  with,  the  kind  consent  of  the  following 
institution: 

National  Library  of  Canada 


L'exemplaire  film6  fut  reproduit  grAce  it  la 
g6n6rosit6  de  I'dtablissement  prdteur 
suivant  : 

Bibliothdque  nationale  du  Canada 


Maps  or  plates  too  large  to  be  entirely  included 
in  one  exposure  are  filmed  beginning  in  the 
upper  left  hand  corner,  left  to  right  and  top  to 
bottom,  as  many  frames  as  required.  The 
following  diagrams  illustrate  the  method: 


Les  cartes  ou  les  planches  trop  grandes  pour  dtre 
reproduites  en  un  seul  clichd  sont  filmdes  d 
partir  de  I'angle  supdrieure  gauche,  de  gauche  d 
droite  et  de  haut  en  bas,  en  prenant  le  nombre 
d'images  ndcessaire.  La  diagramme  suivant 
illustre  la  m6thode  : 


1 

2 

3 

1 

2 

3 

4 

• 

• 

BULLETIN   OF  THE   GEOLOGICAL  SOCIETY   OF    AMERICA 
Vol.  6,  pp.  297-304,  pls.  13-15 


HONEYCOMBED  LIMESTONES  IN  LAKE  lUIRON 


BY 


ROBERT  BELL 


^ 


\  .-?■--■' 


ROC  ULSTER 

PUnLISIIEI)  BY  THE  SOCIETY 

Makch,  1895 


BULLETIN    OF    THE    GEOLOGICAL    SOCIETY    OF    AMERICA 
Vol.  6,  pp.  297-304,  PIS.  13-16  MARCH  25,  1896 


IIONEYCOMREI)  LIMESTONES  IN  LAKE  HURON 

BY    IWnKKT   niCLL 

(Read  before  ihe  Society  December  20,  ISOJf) 

CONTKNTS 

Area  of  oocui  ronop  and  conditions 2!)7 

Pliywical  characteriKtics  of  {\w  orodt'd  rocks 2!>S 

As^c  and  attitude  of  the  eroded  rockH . .    21M) 

Jlronion  forms  in  relation  to  variety  of  rock 2^X) 

Possible  oriijins  of  the  erosion .'100 

Itorinjis  of  niolhisks ."><K) 

Action  of  spontres  ami  al.ua' 1)01 

Wearing  action  of  pehhles .'501 

Acid  water  the  probable  canse  of  the  (Tosion ;i02 

Sulphuric  acid  in  the  water  and  its  source :?0.'t 

Conclusions ;)04 


AnKA   OF   OCCUKHKNCK    AND    CONDITIONS. 

The  limcstonos  in  the  bottom  of  a  certain  jjortion  of  l:il<c  Huron  are 
undergoinj^  a  peculiar  Icind  of  erosion,  which,  from  want  of  l)etter  terms 
to  describe  the  process,  may  be  called  honeycombinjj;  and  pittin«f. 

The  portion  of  the  lake  in  which  this  phenomenon  is  most  prevalent  is 
that  around  Grand  Manitoulin  island,  the  Indian  jx'ninsula  and  in  Big 
gap,  which  lies  between  them  and  connects  the  main  body  of  the  lake 
with  Georgian  bay.  This  curious  form  of  erosion  appears  to  be  progress- 
ing most  rai)idly  under  a  considerable  depth  of  water,  siiy,  from  oO  or 
100  feet,  down  to  greater  depths,  but  it  may  also  be  going  on  in  shallower 
water.  The  existence  of  the  honeycombed  lime^tono  all  over  the  bottom 
of  this  part  of  lake  Huron  is  well  known  to  every  one  living  in  the 
vicinity,  and  es]»eciidly  to  the  fishermen,  to  whom  it  is  a  great  source  of 
annoyance  from  their  nets  catching  upon  it.  Visitors  carry  away  spe(!i- 
mens  of  it  every  sunnner  as  curiosities,  luit,  so  far  as  the  writer  is  aware, 
no  one  has  yet  described  its  occurrence,  attempted  to  explain  its  cause, 
or  reported  a  similar  condition  elsewhere.  The  phenomenon  therefore 
appears  to  be  rare,  if  not  unifpie,  and  worthy  of  a  description  from  a 
geologic  standj)oint. 

XLII-UuM,.  (iEiii..  Hoc.  Am.,  Vol.  0,  1894.  (297) 


298         R.    P.KLL — irOXKYCOMI'.ia)    I.IMKSTONRS    IN    LAKF.    IIUISOX. 

Cinind  Manitoulin  island  is  .SO  milcrt  in  hni^tli.  Al()ii<i  its  soutlicrn 
sidi!  tho  uudistAn'lxid  Silurian  liinostoncs  slope  vorv  <!;('ntly  southward 
under  tlio  lake,  so  that  shallow  water  ext(Mids  a  eoiisideralde  distance 
from  shore.  On  surfnces  which  have  heen  lon<f  exposed  to  the  weather 
or  to  the  wearinjj;  action  of  the  waves  the  pittinfj;  is  more  or  less  cll'aced, 
hut  wherever  they  have  l)een  covered  hy  the  water  or  otherwise  sheltered 
it  is  still  plainl}-  visilile.  In  the  autunni  the  whitefish  come  into  shallow 
water  to  spawn,  and  the  lishermen  say  that  the  rough  limestone  hottoni, 
here  descrihed,  is  a  favorite  resort  for  this  purp<ise.  As  its  ine(|ualities 
l)roteet  the  eggs  from  destructive  currents  and  predaceous  lishes,  it  may 
1)0  said  to  servo  an  economic  purpose,'. 

Physical  ('MAUACTKiiisTKs  ok  tmk  kuodkd  Rocks. 

Various  kinds  of  limestones  have  heen  acted  upon  hy  this  peculiar 
form  of  erosion.  Tho  unaltered  varieties  have  heen  completely  riddled 
with  cavities,  varying  from  very  small  holes  up  to  three  inches  or  more 
in  diameter,  the  average  heing  between  half  an  inch  and  an  inch,  hut  in 
altered  or  crystalline  limestones  or  dolomites  the  pits  arc;  mostly  larger  and 
shallower.  The  jiitting  (tf  these  rocks  will  he  more  fully  noticed  further 
on.  In  unaltereil  dolomites  one  form  assumed  h}'  the  cavities  is  glolmhir 
to  pear-shaped,  iind  in  tlu;  i»roce.ss  of  enlargement  they  encroach  u|ion 
one  ani>ther  until  oidy  thin  walls  remain  between  tliem.  while  others 
coalesce,  and  ultimately  the  whole  mass  becomes  separated  into  a  highly 
erode<l  skeleton,  as  shown  in  ligurcs  I  and  2,  plate  b'i,  taken  from  jihoto- 
graphs.  Even  in  this  stage,  the  solid  angles  between  adjacent  cavities 
become  i)erforate<l  by  smaller  holes,  and  at  length  the  rock  crund)les  to 
fragments,  with  di-eply  hollowed  concave  surfaces.  The  removal  of  such 
a  wast(!d  exterior  exposes  a  ])itted  surface  on  tho  next  lower  layer  of  lime- 
stone, tlu'  contiguous  cu]t-shaped  hollows  usually  occupying  the  whole 
area.  A  completely  sculptured  surface  resembling  this  may  also  be  pro- 
duced by  tho  direct  solvent  action  of  the  water  without  the  intervention 
of  the  glol)ular  honcycondiing.  Large  areas  showing  surfaces  of  this 
kind,  which  had  been  eroded  at  a  consideraltle  depth,  may  now  bo  nwM 
in  the  clear,  shallow  water  or  exposed  to  the  air,  owing  to  tho  lowering  of 
the  level  of  tho  iakc,  while  wide  borders  of  nearly  horizontal  limestone 
beds,  similarly  eroded,  are  exposed  in  some  localities  between  tint  mar- 
gin of  the  water  and  the  wooded  shore.  Those  extremely  eroded  sur- 
fiices  have  a  striking  appearance,  and  the  sharp  and  pointecl  edges  of  tho 
pits  would  be  painful  to  walk  on  without  thick  soled  boots. 

Another  form  which  these  cavities  take  is  finger-shaped,  crowded  to- 
gether and  dce]»ly  indented,  or  of  such  a  shape  as  would  be  completely 
filled  by  inserting  th((  sharp  end  of  a  cigar.  As  in  the  other  forms,  the 
cavities  at  their  tops  adjoin  each  other,  leaving  no  space  between  them. 


BULL.  GEOL   SOC.  AM.                                                                       VOL 

.  b.   1894,  PL.   13. 

111 

7   ■ 

|-'l<.li;l      1  \    I  I  W      \l      \     IM'.III      W'.ll      In     Mil      l>ll>-l']\M. 


l''ii;i  i;i    J— N'litt    m    -  \  mi    Spi  ■  nil  \    \\    \\    .\\i.m    ^i    1..'i,i    im:.   Hi  I'  ii  vni-:. 


HDNtVCOMBtD   DiiLOMITE. 
l-'rom  ."iollth    r,;iy   Mnulll.  M:ilKltimllll    l-l.-llhl.  I  llllirin,  (■.■ill;iil:l 


I 


BULL.  GEOL.  SOC.  AM. 

VOL.  6,  1894.  PL.  t4. 

a^  ■■ 

^ 

'Hi^--?^U' 

sS 

'^^"^ 

BP^mB 

^^^^£SiP 

■  \> 

W^  ,.^^^Kw'j 

^8K '  ''*               ^itKfj^  \ 

• 

w" 

> 

*-*^^i^^ 

I'i'.i  HI-,  1.     \'ii;\\    iT  I  M>n:  ~mumi    \i    v   i.'I'.iii    .\\..ii.   iw  iiii.    l>ih-ri»M;. 


'"I'.i  III    1.    -\irw  "I'  -iMi    ■~1T'LMIN    \\    \\    As'.i.t.  ni    I')     II.   IIII    r>i  ii-ri  \\r.. 


PiTTCtJ  LIMESTONE   OF  THE    BLACK   RIVER   FORMATION. 
[■'I'l'in  l.ittlr  (liHlir  l«;;iii,l,  I  iiii:iri,,.  (^ihaiJM. 


CIIAUACTEUISTICS,   MiK    AND    ATTIirDK.  21)5) 

Tlii.H  variety  of  deeply  pitted  surfaec  rcHemltkw  limestone  wlii(;li  has  been 
tlioniiijilily  riiMled  liy  the  l>urn»\v.s  itf  Sdrirani.  In  a  well  marked  ease, 
to  lie  airain  noticed,  where  a  suhmerj.'ed  shelf  or  hani,dn^  ledj^e  was  ero(led 
on  the  und''r  side,  the  pittinji  look  the  form  hero  deseril>ed.  It  is  well 
shown  in  (ignres  1  and  2,  plate  14. 

A(ih:  ANM  ArnTrDK  ok  rnK  i:Koni:i)  Rocks. 

The  rocks  of  (irand  Manitoulin  and  the  adjacent  islands  end)race  eij.dit 
dill'erent  formations,  from  theChazy,  in  the  nortliern  partofthe  La  Cloche 
island,  to  the  (lUelph,  on  Fitzwilliam  island  and  on  the  southeastern  part 
of  the  main  island,  wiille  dolomites  of  the  Huronian  systen)  are  met  with 
on  many  of  the  islands  of  the  channel  hetwecn  the  Manitoulin  chain 
and  the  nnvin  north  shore  of  lake  Huron.  TlKiChazy  is  rci)resented  l»y 
iirownish  red  and  green  marls  and  line  grained  white  sandstone;  the 
Black  River  consists  of  pure  limestone  and  yellow  weathering  dolomites  ; 
the  Trenton  priiicMpally  of  hluish  gray  limestone,  with  earthy  beds;  the 
litica  of  Mack  hituminous  shale;  the  Hudson  Iviver  of  marls,  with  thin 
iteds  of  limestone  and  sandstone;  the  Clinton  of  dolomite,  with  bright 
red  and  green  marls  at  tlie  base,  while  tlu;  Niagara  and  (iuelph  forma- 
tions are  couiposed  almost  entirely  of  dolomites.  The  dip  is  'uiiforndy 
to  the  south,  at  a  very  low  angle,  and  the  nakeil  beds  of  the  higher  forma- 
tions al»ove  en>imerated  slope  gently  undisr  the  lake  along  the  southern 
sides  of  all  the  islands  of  the  Manitoulin  group. 

Euosiox  Forms  iv  Rki.atio.n  to  Vahiktv  ok  Rock. 

The  largest  and  most  irregular  cavities  are  in  the  magnesian  lime- 
stones of  the  (lUelph  formation.  Their  appearance  //;  ailn  is  shown  in 
l)late  1"),  w'lich  is  from  a  photograj)!!.  The  globular  and  pear-shaped 
varieties,  shown  in  figures  1  and  2,  i)late  13,  are  excavated  in  somewhat 
argillaceous  dolomites  of  the  Niagara  and  Clinton  formations.  The  cup- 
shaped  hollows  are  conunonest  in  the  pure  limestones  (jf  the  Hudson 
River.  Trenton  and  Black  River  and  in  tin,'  dolomites  of  the  last  men- 
tioned tbrmation.  The  finger-shaped  honeycombed  structure  was  found 
principally  in  the  pure  limestone  of  the  Jilack  River  formation,  while  the 
smoother  and  rather  larger  excavations  are  characteristic;  of  the  Huronian 
dolomites.  Tlu;  shallower  varieties  of  this  form  >if  pitting  resemble  that 
of  a  well  eroded  aerolite.  Tlie  occurrence  of  these  various  forms  of 
hone3'combing  and  pitting  in  such  a  variety  of  limestones  and  dolomites 
in  this  portion  of  lake  Huron  proves  that  the  phenomenon  is  not  due  to 
anything  unusual  in  the  general  composition  or  to  any  ch''mi(>al  pecu- 
liarity of  a  particular  variety  of  rock,  but  to  some  outside  cause.  The 
various  forms  which  the  erosion  takes,  however,  show  a  slightly  une(|ual 
solubility  connected  in  some  way  with  the  internal  structure  of  the  rock; 


i 


300         IJ.    I!i:i,[, — IIONKYCOMnKI)    LIMKSTON'KS    FN    I.AKK    HURON'. 

otherwise  it  would  he  of  a  mon;  uniforiu  chivnicter,  .since  it  is  tliie  to  some 
coinmon  extoniiil  cause  operatiujr  alike  upon  them  all. 

The  (lissolviuff  of  the  unaltered  limestones  or  dolomites  <roos  on  at 
right  antdi^s  to  the  l)eddiny;  or  dinictly  downward  or  upward,  as  tl>e  rocks 
are  practically  horizontal,  never  at  an  ohli(piu  an;j;le  nor  horizontally, 
which,  in  the  ahscnco  of  some  inhihitinj;  cause,  mi<fiit  easily  take  place 
in  loose  masses  which  lie  at  all  an<,des  on  the  hottom  of  the  lake.  On 
is()lat(!d  l)Io(!ks,  where  the  sides  have  heen  as  freely  exposed  to  the  water 
as  the  top,  the  solvent  process  appears  to  prefer  to  work  downward  from 
the  "(juarry  lied  "  and  n(jt  to  eat  inwartl  from  the  sides. 

Ex[)eriments  made  hy  Professor  (Joodwin,  of  (Queen's  University, 
Kingston,  on  the  action  of  solvents  on  slowly  soluhle  suhstances  seem  to 
show  that  the  tendency  of  solution  is  strongest  directly  upward  and 
downward.  This,  together  with  a  faint  concretionary  structure,  to  he 
noticeil  further  on,  may  help  to  account  for  thi'sc  lurnis  of  erosion. 

Allusion  has  heen  made  to  a  well  marked  case,  when,'  the  pitting  has 
progressed  directly  upward  from  the  under  surfaces  of  heds  of  jture  lime- 
stone forming  the  roof  of  an  overhanging  ledge  or  shelf  which  had  heen 
suhmerged  when  the  lake  was  a  little  higher  tiian  it  is  at  i»resent.  This 
occurs  along  the  east  side  of  The  Narrows  hetween  Little  Cloche  island 
and  the  southern  part  of  Cloche  peninsula.  The  rock,  which  lulongs  to 
the  Black  Hivcr  formation,  consists  of  a  soft  hluish  grey  limestone  con- 
taining a  little  argillaceous  matter.  The  tapering  finger-shaped  ]»its  are 
closely  crowdeil  together  and  they  penetrate  upwards  i'nini  two  to  four 
inches  from  the  general  outline  of  the  roof.  Silicilied  fossils  project 
from  the  walls  into  these  pits,  and  when  the  rock  is  hroken  across  a  dis- 
coloration of  iron  oxide  is  seen  to  extend  a  short  distance  all  round  the 
wall  of  each  one.     I'late  14  illustrates  these  pits. 

PCSSIBM':  OlUOINS  Ol'  TMK  Euosio.v. 
noiii.yas  of  mollvsks. 

Tilt!  writer  may  not  have  arrived  at  a  (Correct  explaimtion  of  these 
curious  forms  of  I'rosion,  hut  he  will  endeavor  to  state  tin;  suggestions 
which  liMve  occurred  to  him  in  regard  to  their  origin,  along  with  a  <Ie- 
scription  of  the  circumstances  connected  therewith.  Many  theories  to 
account  for  thest;  phenomena  jnay  prestMit  themselves  hoth  to  those  who 
have  and  those  who  have  not  seen  th.jm  on  the  ground,  and  it  may  he 
as  well  here  to  notice  hrielly  the  more  ohvious  ones. 

('iiviti(!S  resendding  some  of  those  ahove  descrihed  ar(>  made  in  rocks 
hy  horing  mollusks,  such  as  Saxlatra,  Phobti^,  Pdricntd,  Vallisin^  Tapex^ 
Vnicnipix  and  I .'lUiodnmnx.  The  case  of  tht;  zones  on  the  niarhle  pillars 
of  the  temple  of  Jupiter  Serapis,  at   I'uzzuoli,  eroded   hy  lithodomi, 


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I'OSSinLK   ORIGINS   OF   TIIK   EROSION.  301 

whicli  has  been  remlercd  clasnic  by  Lyell  in  his  "  Principles  of  Geol- 
ogy," will  occur  to  every  geologist.  The  cavities  formed  by  boring 
nioUusks  are,  however,  burrows  which  are  of  pretty  uniform  calibre,  or 
increase  in  diameter  with  the  depth,  owing  to  the  growth  of  the  animal  as 
it  proceeds.  They  are  generally  many  times  deeper  than  wide,  whereas 
those  under  consideration  are  either  globular  or  short  by  comparison, 
and  they  contract  instead  of  enlarge  toward  the  bottom. 

ACTION  OF  SroyOES  AND  ALOJE. 

A  small  sponge,  Cliona  cclatn,  makes  burrows  in  the  shells  of  oysters, 
and  this  fact  led  to  the  supposition  that  possibly  some  species  of  fresh- 
water sponge  might  have  aided  in  deepening  or  enlarging  the  cavities 
under  discussion,  or  in  determining  their  form  by  the  production  of 
small  quantities  of  an  organic  acid,  either  during  life  or  upon  decompo- 
sition, but  no  evidence  could  be  found  in  support  of  this  idea.  The  re- 
mains of  branching  fresh-water  alga?  may  be  seen  in  some  of  the  cavities, 
but  they  do  not  appear  to  have  exercised  the  least  influence  in  their 
l)roduction.  The  lower  or  jelh'-like  algai  of  fresh  water  do,  however, 
possess  the  power  of  dissolving  limestone.  The  journal  of  the  Royal 
Microseo})ical  Society  for  October,  1894,  says,  on  page  597,  that — 

"  ProfoHHor  F.  Cohn  i)oint8  out  tlie  important  part  i)laye(l  by  very  lowly  organ- 
ized aljiii' — PhycochroinaooiC  and  Cyanophycoje — in  the  formation  of  calcareous 
and  KiliciouH  rocks.  Many  beds  of  marl)lo  and  travertine  liave  heen  formed  in 
this  way.  He  furtlier  enumerates  the  algje  that  are  known  to  de.stroy  calcareous 
rocks  by  erosion.  In  all  fixed  liV^ve  there  appears  to  ))e  this  contrast  between  the 
basal  cells  and  the  rest  of  the  Hlament;  that  the  former  excrete  an  acid  which 
dissolves  lime,  while  the  laiter  has  the  power  of  depositing  a  soluble  lime-salt  be- 
tween the  iilaments,  but  within  the  mucilage  which  is  e.\creted  from  the  sheath." 

WEARING  ACTION  OF  PEDDLES. 

A  i)opular  notion  current  among  many  of  the  residents  of  the  localities 
where  the  pitted  surfaces  occur  is  that  the  cavities  have  been  worn  by 
the  whirling  of  itebbles  and  sand  in  a  maimer  analogous  to  the  forma- 
tion of  potholes  at  rapids  and  falls.  That  they  have  not  been  thus  formed 
is  obvious  from  the  following  reasons,  selected  from  among  others: 

Their  sha])es  do  not  corresi)ond  with  this  mode  of  formation. 

They  occupy  the  whole  surface  of  the  rock,  whereas  iwtholes  occur  at 
irregular  int(;rvals. 

The  walls  of  the  cavities  are  generally  uneven  or  rough,  and  delicate 
silicified  fossils  often  project  from  them  or  stretch  completely  across  the 
cavities,  whereas  the  wearing  action  of  rotating  pebbl(\s  and  sand  would 
have  produced  smooth  and  cylindrical  walls. 

No  pebl)les  are  ever  found  in  tluMn  except  such  as  can  be  shown  to 
have  been  placed  there  subsetiuent  to  their  excavation.     If  these  cavities 


302         R.  15KLL — HONEYCOMIJliD    LIMKHTONKS   IN    LAKK    III'IIOX. 

had  ])een  due  to  tliis  rauso,  tluiir  occurronco  would  uotbo  coufincMl  to  tlie 
liiuestont's  of  lake  Huron,  l>ut  would  l>o  a  <fcMU'ral  phononieuou  in  cou- 
ntH'tion  witli  similar  rocks  all  over  the  worhl.  On  the  contrary,  the 
pel)hlo  theory  is  not  sustained  by  facts  anywhere.  I'el)l)les  washed  by 
currents  passing  over  rocks  or  <^rated  against  them  h}'  the  waves,  not  only 
do  not  pit  or  honeycoinl)  rocks,  hut  they  have  the  o[»p()site  effect  and 
wear  them  snujotii. 

It  is  true  tiiat  gravel  may  now  he  seen  scattered  over  local  pitted  sur- 
faces wiiich  have  heen  laid  h;ire  hy  the  recession  of  the  lake,  but  on 
examining  cases  of  this  kind  it  is  always  manifest  that  the  gravel  was 
jdaced  upon  such  surfaces  long  after  the  completion  of  the  pitting. 

The  occurrence  of  the  cavities  equally  on  the  tops  of  loose  blocks  of 
limestone  and  on  the  solid  beds  is  another  objection  to  this  theory. 

A  final  objection  is  the  fact  tlmt  some  of  the  most  beautiful  examples 
of  deep  pitting  are  found  on  the  under  surfaces  of  overhanging  beds,  the 
eroding  agent  having  worked  U{)ward  in  a  free  body  of  water. 

Acid  Waticr  tiik  i'kobaih.k  Caisk  ok  tiik  Erosion. 

Having  eliminated  all  the  possible  causes  which  have  suggested  them- 
Helves  as  being  unlikely  to  have  produci'd  the  erosion  un<ler  discussion, 
the  cpicstion  arises  :  To  what  must  weattrilsuteit?  It  appears  most  ]>rob- 
able  that  the  cause  will  be  found  in  the  dillcrcntial  solubility  of  t\u)  rock 
in  the  water  of  the  lake  itself.  The  slight  difference  in  the  solubility  of 
those  parts  of  the  rock  which  give  rise  to  the  cavities  is  likely  due  to  its 
internal  structure,  which  originated,  in  the  first  place,  at  the  time  of  the 
formation  of  the  limestones  or  dolomites  themselves.  The  eroded  beds 
are  not  generally  those  which  are  largely  made  uj)  of  organic  remains, 
but  oftener  those  which  have  been  due  to  chemical  precipitation.  Then^ 
are,  however,  excei)tions  to  this  general  rule  which  add  to  the  difiiculty 
o*'  accounting  for  this  singular  i)henonu'n()n.  In  the  process  of  consoli- 
dation of  chemically  formed  beds  in  which  Hiik;,  magnesia  and  argilla- 
ceous matters  were  present,  there  would  naturally  be  more  or  less  tend- 
ency to  concretionary  action  around  certain  points  or  centers,  thus  giving 
ris(!  to  slight  differences  in  composition.  Kvcn  if  such  cotKtretionarv 
structure  were  too  obscure  to  be  readily  noticed,  the  probability  of  its 
existence  in  the  rocks  of  the  al»ove  composition  will  be  readily  admitted ; 
but  in  some  cases,  especially  where  oxide  of  iron  is  present,  this  structure 
may  be  detected  in  the  form  of  obscure  concentric  lines.  The  globular 
shape  of  the  cavities  is  a  fact  which  points  to  this  origin.  Hut,  if  this  bo 
true,  it  may  be  asked,  why  are  similar  linu!stones  not  always  eroded  in 
this  maimei  when  they  are  covered  by  fresh  water?  The  ijxplanation 
of  the  difference  is  probably  to  be  found  in  a  sufficiently  acid  condition 
of  the  water  of  this  part  of  the  lake  to  slowly  dissolve  the  limestones. 


ACID   WATKR    AND    ITS    KFFECTS.  303 

The  solvent  action  of  tlie  sli<j;litly  acidulated  water  is,  no  doubt,  aided  by 
certain  conditions  fnvoraltle  to  it  in  the  present  case,  hut  which  nii^iit  he 
al)sent  elsciwlien^,  even  if  tlie  rccjuisite  amount  of  acidity  existed.  The 
water  of  tiiis  i)art  of  lake  Huron  is  perfectly  limpid  and  free  from  sus- 
pended impurities,  so  that  there  is  nothinj?  to  check  the  proj^ress  of  the 
dissolvinjf  process,  no  matter  how  slowly  it  may  l)e  p;oin<j;  on.  That  it  has 
been  exceedin,<rly  slow  in  its  operation  is  shown  by  the  pitted  or  honej'- 
combed  surfaces  retaining  their  glaciated  forms.  As  the  basin  of  lake 
Huron  has  no  doubt  been  filled  with  water  since  the  disappearance  of  the 
ice-sheet,  the  time  retjuired  for  the  erosion  imder  consideration  must  have 
been  many  thousands  of  years,  but  the  proportion  of  acid  in  the  water 
probably  increased  gradually,  as  the  explanation  to  lollow  will  show. 

Sulphuric  A(;id  ix  the  Water  and  its  Source. 

Although  the  water  of  this  part  of  lake  Huron  has  not  been  analyzed, 
it  ajtpears  to  contain  a  notaltle  pro])ortion  of  sulphuric  acid.  It  has  the 
l)roperty  of  slowly  reddening  vegetable  matter,  purple  or  blue.  Its  cor- 
roding action  on  tin  pails  and  pans  is  a  source  of  annoyance  and  loss  to 
housekeepers  and  campers  who  use  the  water  for  domestic  j>urposes.  A 
new  tin  pail,  if  kept  filled  with  the  lake-water,  will  in  a  few  days  show 
rusty  excrescences  in  the  bottom,  which  increase  rapidly  and  soon  per- 
forate the  vessel.  Tlie  water  of  the  portion  of  the  lake  where  honeycoml)- 
ing  of  limestones  is  most  obvious  is  distinctly  harder  than  elsewluire, 
probably  owing  to  the  ])ressure  of  sulphate  of  lime. 

'i'he  source  of  the  sulphuric  acid  may  be  looked  for  in  the  Huronian 
rocks  lying  to  the  nortlnvard  of  the  lake.  It  is  nMuarkable  that  tlie  por- 
tion of  the  lake  in  whicli  this  form  of  erosion  of  limestt)ne  takes  place 
lies  directly  in  front  of  that  part  of  the  north  shore  occupied  by  the 
Huronian  rocks,  and  receives  several  consideral»le  rivers  which  drain  an 
immense  area  of  rocks  of  the  same  age.  They  are  mostly  of  volcanic 
origin  and  arc  rich  in  sul[)hi(les,  whereas  all  the  other  rocks  around  the 
lake  are  comparativel}'  free  from  them. 

Sulphides  of  several  metals,  but  especially  of  iron,  are  disseminated 
through  most  of  the  Huronian  rocks.  Pyrites  and  pyrrhotite  are  par- 
ticularly al»undant  in  the  greenstones,  and  these  form  a  considerable 
j)roportion  of  the  series.  The  two  suli)hides  referred  to  occur  both  as 
disseminated  grains  and  in  the  form  of  numerous  masses,  some  (juite 
large,  such  as  those  around  Sudbury,  a  few  of  which  are  being  worked 
for  nickel.  The  decomposition  of  these  sulphitles  upon  the  surface  of 
the  rocks  and  along  their  fissures  and  joints  produces  iron-sulphate, 
which  is  carried  away  by  the  streams,  where  its  ])resence  is  fre(]Uently 
shown  by  the  i)recipitation  of  a  part  of  the  iron.  In  the  swamp  at  the 
Murray  mine  near  Sudbury,  where  a  large  mass  of  pyrrhotite  and  dial- 


1 


304         K.   BKLL — HONICYCOMBED    LIMESTONES   IN    LAKE   HURON. 

copyrito  comes  to  the  surface,  the  i)resence  of  acid  sulphate  could  be 
detected  by  the  taste  of  the  water,  and  when  the  latter,  containiufi;  as  it 
does  vegetable  matter  also,  was  used  experimentally  for  the  boilers,  it  gave 
off  a  most  offensive  smell  and  had  a  very  corrosive  eff'ect.  The  streams 
of  the  whole  of  this  Huronian  region  no  doubt  receive  many  contribu- 
tions of  acidulated  water  from  similar  sources.  As  the  decomposition  of 
the  greenstones  and  other  pyritiferous  rocks,  and  also  the  oxidation  of 
the  drift  materials  derived  from  tliena,  j)roceeds,  the  quantity  of  acid 
derived  from  them  and  carried  into  the  northern  part  of  lake  Huron 
will  increase.  At  The  Narrows  between  Cloche  peninsula  and  Little 
Cloche  island,  as  already  mentioned,  the  pits  in  the  soft  argillaceous 
grey  limestone  are  surrounded  by  a  zone  stained  by  iron  oxide.  This  is 
just  what  might  be  expected  to  result  if  such  a  rock  were  being  slowly 
dissolved  by  iron-sulphate  and,  therefore,  this  fact  helps  to  support  the 
present  hypothesis. 

Tiie  fresh  rock-surfaces  and  uuoxidized  drift  left  at  the  close  of  the 
Glacial  epoch  would  produce  a  much  smaller  proportion  of  sulphuric 
acid  in  a  given  time;,  and  we  therefore  supi)ose  that  the  erosion  of  the 
limestones  in  the  bottom  of  the  northern  part  of  lake  Huron  went  on 
even  more  slowly  then  than  now. 

Another  example  may  be  cited  here  of  water  containing  sulphuric 
acid  which  has  been  derived  api)arently  from  volcanic  rocks.  A  sample 
from  the  shallow  fresh-water  in  the  estuary  of  the  Nelson  river,  Hudson 
bay,  was  taken  by  the  writer  to  the  late  Professor  William  Dittmar,  the 
well  known  authority  on  water  analyses,  and  he  found  it  to  contain  no 
less  than  4.73  grains  of  sul()huric  acid  to  the  imperial  galhju.*  The 
source  of  this  acid  appeared  to  be  the  drift-material  which  had  come 
from  the  volcanic  rocks  of  the  central  or  eastern  part  of  the  bay. 

Conclusions. 

The  conditions  which  have  contril)uted  to  the  production  of  the  pecu- 
liar forms  of  erosion  al)Ove  described  ai)pear  to  have  been  : 

1.  The  internal  structure  of  the  limestone  itself. 

2.  A  small  {^upjitity  of  acid  in  the  water  acting  for  a  great  length  cf 
time. 

3.  A  considerable  depth  of  water,  the  hydrostatic  pressure  seeming  to 
promote  the  dissolving  of  the  rock. 

4.  Freedom  from  sediment  during  the  long  time  required. 

5.  The  rock  must  be  exposed  to  the  open  or  free  action  of  the  water. 

6.  Shifting  currents  in  the  water  would  also  appear  to  assist  the  process. 

•See  Appendix  V,  Report  C,  of  the  Cniuuliiin  Geological  Survey  for  1879-80,  p.  78. 


